The present invention relates to magnetic contact switches, and particularly to a switch assembly which may be assembled in alternative configurations to provide either normally closed or normally opened contacts.
Magnetic reed switches are normally capable of millions of cycles of operation before failure, since the contacts are not exposed to corrosive elements of the atmosphere and there are no bearings to fail. Encapsulated magnetic contact reed switches are often used for security systems and in position sensing applications, because of this longevity and reliability of such magnetic reed switches.
Use of such switches as proximity switches usually entails use of the magnetic field of an actuating magnet which is movable with respect to the switch itself, the actuating magnet being attached to one of two relatively movable objects, while the switch itself is mounted on the other. Such a separately mounted movable actuating magnet, however, must generally be considerably larger, and thus more costly, than the smallest magnet which would be required to actuate the same magnetic reed switch if placed immediately adjacent to the switch capsule.
An additional disadvantage to the use of an actuating magnet mounted separately on a moving part is that the sensitivity of the particular magnetic reed switch is an important factor in the performance of the security system or position monitoring system utilizing a magnetic reed switch actuated in this manner. Since a more sensitive magnetic reed switch can operated by a given actuating magnet at a greater distance, closely similar operating characteristics can be provided in a number of switch assemblies only by using switches of closely similar characteristics.
Because of the high cost of stable permanently magnetic material, it is economical to use the longevity of the reed switch by providing a small actuating magnet movably mounted in close proximity to the magnetic reed switch itself, along with a suitable mechanism, such as a plunger moved by mechanical contact with the object whose position is to be detected by the switch, for moving the actuating magnet.
While some use of this principle has been made in the past, as illustrated in Mayer, U.S. Pat. No. 3,243,544, and Yokoo, U.S. Pat. No. 3,260,821, the switch assemblies disclosed in those patents have disadvantages which make them unsuitable for use in security systems and certain other position sensing applications. For example, the construction of the Mayer switch would appear to make it vulnerable to wet weather, and the number and shape of magnets required in the Yokoo device make it unnecessarily expensive.
It is desirable in a magnetic reed switch assembly to have the option of using a single type of magnetic reed switch having a pair of contacts which are normally open, in either a normally open circuit or a normally closed circuit. The mechanism disclosed by Mayer for this purpose, however, is unnecessarily complex and susceptible to damage. In particular, the use of springs surrounding the magnetic reed switch capsules, as in the Mayer device, may provide undesirable magnetic field influence on the magnetic reeds. Additionally, the Mayer device requires the use of toroidal magnets fitting around the magnetic reed switch capsule, making the magnets unnecessarily expensive.
Yokoo discloses a magnetic reed switch assembly having a push rod operated carrier for moving an actuating magnet in a direction parallel with the length of the reeds of the reed switch to actuate the magnetic contacts. Yokoo, however, does not disclose any provision for assembly of the switch alternatively in normally open or normally closed contact modes. Additionally, the Yokoo switch requires an undesirably large amount of magnetic material, depending on the action of a pair of mutually attractive magnets to restore the actuating magnet to a normal position.
What is desired, then, is a switch assembly incorporating a single form of magnetic reed switch which may be utilized in either a normally open or a normally closed mode, which requires only a small permanent magnet, and which is simple in construction and manner of assembly.